Therapeutic options for patients with recurrent leukemia or B cell lymphoma refractory to the standard therapy are limited. A promising approach for such resistant malignancies is immunotherapy with e.g. donor lymphocyte infusions (1-3). Donor lymphocyte infusions can cure especially recurrent chronic myelogenous leukemia, probably by the action of T cells, but is complicated by the possibility of simultaneously induced graft-versus-host disease. Therefore, a more specific and better controllable therapy, e.g. with monoclonal antibodies that target lymphocytes to residual tumor cells, is desired. However, in most cases, unlabeled or unconjugated antibodies do not activate immune effector mechanisms sufficiently to eradicate all tumor cells. Thus, T cell redirecting bispecific antibodies (bsAb), which combine the cell-specificity of monospecific antibodies with the potency of T cells may be more effective.
Bispecific antibodies consisting of 2 different antigenic specificities have been developed as immunotherapeutic reagents for targeting immune cells to tumor tissue (4-8). This strategy is based on the assumption that appropriate effector/target cell interaction via a physical contact between immune cells and tumor cells, activates cytotoxic mechanisms that lead to an efficient eradication of tumor cells.
Many different bispecific antibody formats have been created over the last 20 years with varying qualities in production, in vitro and in vivo efficacy, recruitment of effector cells or e.g. need for additional T cell stimuli (9-13). Finally, several of these constructs reached the clinic with minor to moderate and only sometimes promising efficacy (14-17). Trifunctional antibodies (trAb) are artificially engineered immunoglobulins with an unique composition of heavy chains of mouse IgG2a and rat IgG2b, representing highly homologous Ig subclasses. Both isotypes are very potent in terms of immunological effector functions, such as complement dependent cytotoxicity (CDC) and antibody dependent cell-mediated cytotoxicity (ADCC). Noteworthy, the Fc region composed by these two subclasses, effectively binds to human FcγI and III-receptors on accessory cells (like e.g. macrophages, dendritic cells and natural killer cells) but not to the inhibitory Fcγ receptor type II expressed e.g. on B cells. As a consequence trAbs can not only redirect T-cells to tumor cells, but also induce recruitment and activation of accessory cells through their Fc region. The simultaneous activation of different mechanisms at the tumor site such as phagocytosis, perforin mediated lysis and cytokine release results in a particularly efficient destruction of tumor cells (18, 19). Remarkably, also apoptosis-resistant tumor cells can be eliminated by this process (19). As a further consequence of the so induced uptake of tumor material by the antigen presenting system even a long-lasting protective antitumor immunity could be established as already demonstrated in two immunocompetent murine tumor models (20).
While most BsAb treatments aiming to achieve anti-tumor response have been combined with syngeneic/autologous derived cells, we and others (3, 10, 21-23) have also used an immunotherapeutic strategy based on the allogeneic reaction of major histocompatibly mismatched cells, known in clinical practice as donor lymphocyte infusion (DLI), following hematopoietic stem cell transplantation (SCT). Unfortunately, the use of allogeneic cell therapy (alloCT) in the context of currently applicable protocols in experimental models and in clinical practice, is frequently accompanied by life-threatening acute and chronic graft versus host disease (GVHD) that is difficult to control effectively with currently available treatments (23-28).